1. Field of the Invention
The present invention relates to a circuit-incorporating light receiving device including a light detection photodiode portion and a circuit element on the same substrate. More particularly, the present invention relates to a structure of the circuit-incorporating light receiving device for improving a performance of the light detection photodiode portion.
2. Description of the Related Art
Recently, optical disk apparatuses are becoming smaller and smaller while the performance thereof is becoming higher and higher. With such advances, there is an increasing need for a compact and lightweight optical pickup. To achieve such an optical pickup, a technology has been proposed in which: a function for generating a tracking beam, a function for branching light, and a function for generating an error signal are integrated onto a single hologram device; a laser diode, a split photodiode, and the like are accommodated in a single package; or the hologram device is provided on an upper surfaces of the package. Such a technology is called an optical module.
Among components included in an optical pickup apparatus is a circuit-incorporating light receiving device. In the circuit-incorporating light receiving device, circuit elements, such as a light detection photodiode portion for converting signal light to an electric signal (optoelectric transduced signal), a transistor for processing the optoelectric transduced signal, a resistor, and a capacitor, are integrated.
FIG. 5 is a cross-sectional view showing a structure of a conventional circuit-incorporating light receiving device 4000.
The circuit-incorporating light receiving device 4000 includes a photodiode region 51 in which a light detection photodiode portion which converts signal light to an electric signal is provided, and a peripheral circuit region 52 which is used to process the optoelectric transduced signal. Specifically, in the peripheral circuit region 52, an NPN transistor and a vertical PNP transistor are provided.
To reduce cost in fabrication of the circuit-incorporating light receiving device 4000, the commonality of fabrication processes is increased. For both the photodiode region 51 and the peripheral circuit region 52, a P-type substrate 53 (P), a P-type epitaxial layer 54 (Pxe2x88x92), and an N-type epitaxial layer 55 (N) are successively provided in this order. In the photodiode region 51, the P-type epitaxial layer 54 and the N-type epitaxial layer 55, which have PN junction, form a light detection photodiode portion. In the peripheral circuit region 52, the above-described two transistors are provided in the P-type epitaxial layer 54 and the N-type epitaxial layer 55 due to impurity diffusion.
The photosensitivity and response speed of a photodiode are generally key measures of the performance of the photodiode. The photosensitivity is determined by the sum of the number of carriers generated in a depletion layer and the number of carriers which are generated outside the depletion layer and which reach the depletion layer due to carrier diffusion when reverse bias is applied to the PN junction in light detection. The response speed is influenced to a large extent by the value of a PN junction capacitance of the light detection photodiode portion. Therefore, to enlarge the depletion layer sufficiently is effective to increase the sensitivity of the photodiode and reduce the junction capacitance to increase the response speed.
Therefore, as a first conductivity type region, the P-type substrate 53, on a surface of which the P-type epitaxial layer 54 having a low concentration (high specific resistance) is provided, is used as described above. Alternatively, a P-type low-concentration substrate (not shown) may be used instead.
Such a structure causes a depletion layer to be easily expanded in the first conductivity type region in which light is absorbed, thereby making it possible to efficiently utilize penetrating signal light. Further, the PN junction capacitance can be reduced.
As the recording density of an optical recording medium, such as an optical disk, becomes higher and higher year after year, the wavelength of light applied to the medium is decreased. Specifically, whereas infra-red light having a wavelength of 780 nm is used for CDs, red light having a reduced wavelength of 650 nm is used for DVDs. The use of blue light having a further reduced wavelength of about 410 nm is being developed.
However, as the wavelength of signal light is reduced, the depth of silicon to which the signal light can reach (hereinafter referred to as a penetration depth) is rapidly decreased. For example, although the penetration depth of 780-nm light is as long as about 8 xcexcm, the penetration depth of 410-nm light is less than or equal to about 0.3 xcexcm.
There are the following problems with the structure of the photodiode region in the conventional circuit-incorporating light receiving device 4000 of FIG. 5.
(1) The N-type epitaxial layer 55 typically needs to have a thickness of at least about 1 xcexcm or more in order to provide a transistor in the peripheral circuit region 52. Further, an N-type diffusion region 56 (N+) having a high concentration is provided in order to reduce a cathode resistance, sot hat penetrating light is mostly absorbed by the N-type epitaxial layer 55 which has substantially no depletion. Therefore, the recombination rate of carriers is high and the recombined carriers cannot contribute to an optoelectric current, so that the sensitivity cannot be enhanced. Further, the PN junction capacitance of the light detection photodiode portion is too large to achieve a high response speed.
(2) The N-type epitaxial layer 55 could be caused to be thin, not taking into account the conformity with the peripheral circuit region 52. In this case, when the N-type epitaxial layer 55 is grown, P-type auto dope occurs due to a peripheral isolation diffusion region 57 (P+) or a film production apparatus. The occurrence of auto dope leads to formation of a potential peak in the vicinity of an interface between the first conductive region (the P-type substrate 53 having the low-concentration P-type epitaxial layer 54 grown thereon, or the P-type low-concentration substrate) and the N-type epitaxial layer 55 grown thereon, which deteriorates a response characteristic.
(3) To reduce the fabrication steps and improve the sensitivity to short-wavelength light, a light detection photodiode portion in which a P-type diffusion region is provided in the N-type epitaxial layer 55 may be provided. However, in this case, boron used in the P-type diffusion region is segregated on a surface thereof, so that Ns (surface concentration) is lowered. As a result, the surface recombination is increased, so that the sensitivity is lowered. Further, the photosensitivity of the light detection photodiode cannot be increased for long-wavelength light having a large penetration depth due to the structure thereof. Since the depletion layer is not so enlarged, the junction capacitance is increased and therefore the response speed is lowered.
(4) In the conventional structure of FIG. 5 in which the light detection photodiode portion is separated by the isolation diffusion region 57 in which diffusion is carried out upward and downward, it is assumed that light is incident to the isolation diffusion region 57. As shown in FIGS. 6A and 6B, the impurity concentration distribution of the isolation diffusion region 57 has a profile taken along a line Axe2x80x94Axe2x80x2 of FIG. 5 and a profile taken along a line Bxe2x80x94Bxe2x80x2 of FIG. 5, respectively. Generated carriers are accumulated in a valley in FIG. 6A. Referring to FIG. 6B, since the middle portion has substantially no tilt, the accumulated carriers laterally move at a low speed. Therefore, the response speed cannot be improved.
According to the above-described reasons, a plurality of circuit-incorporating light receiving devices corresponding to respective wavelength regions are required in a recording and reproduction apparatus in order to be compatible with optical recording media using signal light having different wavelengths, such as a short wavelength and a long wavelength. This leads to a complex system.
According to one aspect of the present invention, a circuit-incorporating light receiving device comprises a first semiconductor substrate of a first conductivity type, a first semiconductor layer of the first conductivity type, a second semiconductor layer of the first conductivity type, a diffusion region of the second conductive type, provided in a first portion of the second semiconductor layer of the first conductivity type, a circuit element provided in the first portion of the first semiconductor layer of the first conductivity type and a second portion of the second semiconductor layer of the first conductivity type. The second semiconductor layer of the first conductivity type and the diffusion region of the second conductivity type form a light detection photodiode portion, and the diffusion region of the second conductivity type has a diffusion depth less than or equal to a penetration depth of short-wavelength signal light.
In one embodiment of this invention, the diffusion depth of the diffusion region of the second conductivity type is less than or equal to 0.3 xcexcm.
In one embodiment of this invention, the short-wavelength signal light is blue light.
Therefore, the thickness of the first and second semiconductor layers in which the circuit element is provided can be sufficient, and PN junction can be provided at a shallow position in a surface of a photodiode region in the second semiconductor layer. Even short-wavelength signal light can be sufficiently absorbed. This leads to an achievement of a circuit-incorporating light receiving device having a high-sensitivity light detection photodiode portion.
In addition, when the concentrations of both first and second semiconductor layers are set to low values, the depletion layer in the light detection photodiode portion sufficiently expands toward a substrate side as does in a conventional case. This leads to an achievement of a circuit-incorporating light receiving device having a high-sensitivity light detection photodiode portion. These performances are not lowered in the case of long-wavelength signal light.
This is particularly effective when silicon is used as a semiconductor material included in the photodiode region.
In one embodiment of this invention, a surface impurity concentration of the first semiconductor layer of the first conductivity type is greater than or equal to 1xc3x971014 cmxe2x88x923.
Therefore, an anti-auto dope ability can be enhanced. Undesired auto dope concentration is about 1xc3x971014 cmxe2x88x923 at most. In the case of the above-described surface concentration, the auto dope does not substantially influence on a variation in a characteristic of the light detection photodiode.
In one embodiment of this invention, the first semiconductor layer of the first conductivity type and the second semiconductor layer of the first conductivity type have impurity concentrations such that when reverse bias is applied to the light detection photodiode portion upon detection of the signal light, a depletion layer expanding from an interface between the second semiconductor layer of the first conductivity type and the diffusion region of the second conductivity type toward the second semiconductor layer of the first conductivity type side, reaches a position deeper than an interface between the first semiconductor layer of the first conductivity type and the second semiconductor layer of the first conductivity type.
This makes it possible to remove an influence of a potential peak at the interface between the first and second semiconductor layers due to the auto dope in the fabrication steps.
In one embodiment of this invention, a high-concentration buried layer of the first conductivity type is provided between the first semiconductor substrate of the first conductivity type and the first semiconductor layer of the first conductivity type, wherein an impurity concentration of the high-concentration buried layer is greater than an impurity concentration of the first semiconductor layer of the first conductivity type.
Therefore, an anode resistance in a traverse direction up to an electrode is reduced. Further, a potential barrier is produced against carriers generated deeper than the buried layer when long-wavelength signal light enters the device. In this case, the carriers are prevented from contributing to a photoelectric current, thereby making it possible to prevent a reduction in a response speed due to a diffusion current.
In one embodiment of this invention, the high-concentration buried layer of the first conductivity type is provided by a buried diffusion method or an epitaxial growth method.
In the case of the buried diffusion method, the circuit-incorporating light receiving device can be easily obtained. In the case of the epitaxial growth method, a concentration profile thereof can be controlled with great precision.
In one embodiment of this invention, the first portion of the first semiconductor layer of the first conductivity type and the second portion of the second semiconductor layer of the first conductivity type have an N-type well region and a P-type well region, and the circuit element is provided using the N-type well region and the P-type well region.
Conditions, such as the conductivity type and concentration of an epitaxial growth layer on which a circuit element, such as a transistor, is provided, can be freely designed for a photodiode.
In one embodiment of this invention, an isolation diffusion region of the first conductivity type is provided between the light detection photodiode portion and the circuit element, wherein an impurity concentration of the isolation diffusion region is greater than an impurity concentration of the first semiconductor layer of the first conductivity type, and the isolation diffusion region of the first conductivity type reaches a position deeper than the light detection photodiode portion and the circuit element.
An anode resistance in a depth direction up to an electrode contact can be reduced, thereby reducing a CR time constant and achieving a high-speed response characteristic.
In one embodiment of this invention, an impurity concentration of each of the first semiconductor layer of the first conductivity type and the second semiconductor layer of the first conductivity type is greater than or equal to 1xc3x971014 cmxe2x88x923.
A circuit-incorporating light receiving device can be achieved which has a light detection photodiode portion which can absorb any light from long-wavelength to short-wavelength, and a high sensitivity and a high-speed response over a range from long-wavelength to short-wavelength.
In one embodiment of this invention, the diffusion region of the second conductivity type is divided into a plurality of regions by the second semiconductor layer of the first conductivity type.
Since an isolation structure obtained by an upward and downward high-concentration diffusion method is not used, substantially no high-concentration isolation diffusion region does not exist. Therefore, substantially no depletion layer occurs in the plurality of regions, and carriers generated immediately under the plurality of regions reach the depletion layer without going around the isolation diffusion region. Thereby, a circuit-incorporating light receiving device having a divided light detection photodiode portion having an improved response speed can be achieved.
In one embodiment of this invention, the diffusion region of the second conductivity type is divided into a plurality of regions by a groove.
A parasitic capacitance of a side can be reduced to more extent than when the light detection photodiode is divided by an upward and downward high-concentration diffusion method using an isolation diffusion region. Thereby, a circuit-incorporating light receiving device having a divided light detection photodiode portion having an improved CR time constant can be achieved.
In one embodiment of this invention, the groove is provided by a LOCOS method.
Since a semiconductor layer is removed by the LOCOS method, a highly-reliable photodiode can be obtained. Although a trench method may be used to fabricate the structure of the present invention, an increase in a junction leak due to dry etching leads to deterioration of a photodiode characteristic. A circuit-incorporating light receiving device having a more reliable light detection photodiode portion can be achieved when the groove is produced by the LOCOS method.
According to another aspect of the present invention, a circuit-incorporating light receiving device comprises a first semiconductor substrate of a first conductivity type, a first semiconductor layer of the first conductivity type, a second semiconductor layer of the first conductivity type, a first groove reaching the first semiconductor layer of the first conductivity type being provided in the second semiconductor layer, a diffusion region of a second conductivity type provided in a first portion of the first semiconductor layer of the first conductivity type, the first portion being exposed at a bottom side of the first groove, and a circuit element provided in a second portion of the first semiconductor layer of the first conductivity type and a first portion of the second semiconductor layer of the first conductivity type. The first semiconductor layer of the first conductivity type and the diffusion region of the second conductivity type form a light detection photodiode portion. The diffusion region of the second conductivity type has a diffusion depth less than or equal to a penetration depth of short-wavelength signal light.
Therefore, conditions, such as the conductivity type, concentration, thickness, and the like of the second semiconductor layer, and be freely determined independently of the light detection photodiode portion. Conditions required for the circuit element can be designed. PN junction can be provided at a shallow position in the surface of the photodiode region in the first semiconductor layer, and even short-wavelength signal light can be sufficiently absorbed in the depletion layer. Thereby, a circuit-incorporating light receiving device which has a circuit element having an optimized characteristic and a high-sensitivity light detection photodiode can be achieved.
Since the depth of the groove reaches a region deeper than the interface between the first semiconductor layer and second semiconductor layer, an auto dope layer generated in the vicinity of the interface can be removed. Further, when the concentration of the first semiconductor layer is set to be a low value, the depletion layer in the light detection photodiode portion sufficiently expands toward a substrate side as does in a conventional case. This leads to an achievement of a high-speed light detection photodiode portion. These performances are not lowered in the case of long-wavelength signal light.
In one embodiment of this invention, the diffusion depth of the diffusion region of the second conductivity type is less than or equal to 0.3 xcexcm.
In one embodiment of this invention, the short-wavelength signal light is blue light.
In one embodiment of this invention, a high-concentration buried layer of the first conductivity type is provided between the first semiconductor substrate of the first conductivity type and the first semiconductor layer of the first conductivity type, wherein an impurity concentration of the high-concentration buried layer is greater than an impurity concentration of the first semiconductor layer of the first conductivity type.
In one embodiment of this invention, the high-concentration buried layer of the first conductivity type is provided by a buried diffusion method or an epitaxial growth method.
In one embodiment of this invention, the second portion of the first semiconductor layer of the first conductivity type an the first portion of the second semiconductor layer of the first conductivity type have an N-type well region and a P-type well region, and the circuit element is provided using the N-type well region and the P-type well region.
In one embodiment of this invention, an isolation diffusion region of the first conductivity type is provided between the light detection photodiode portion and the circuit element, wherein an impurity concentration of the isolation diffusion region is greater than an impurity concentration of the first semiconductor layer of the first conductivity type, and the isolation diffusion region reaches a position deeper than the light detection photodiode portion and the circuit element.
In one embodiment of this invention, an impurity concentration of the first semiconductor layer of the first conductivity type is greater than or equal to 1xc3x971014 cmxe2x88x923.
In one embodiment of this invention, the diffusion region of the second conductivity type is divided into a plurality of regions by the first semiconductor layer of the first conductivity type.
In one embodiment of this invention, the diffusion region of the second conductivity type is divided into a plurality of regions by a second groove.
In one embodiment of this invention, the first groove is provided by a LOCOS method.
According to another aspect of the present invention, a circuit-incorporating light receiving device comprises a first semiconductor substrate of a first conductivity type, a first semiconductor layer of the first conductivity type, a first groover reaching the first semiconductor substrate of the first conductivity type being provided in the first semiconductor layer, a diffusion region of a second conductivity type, provided in a first portion of the first semiconductor substrate of the first conductivity type, the first portion being exposed at a bottom side of the first groove, and a circuit element provided in a second portion of the first semiconductor substrate of the first conductivity type and the first portion of the first semiconductor layer of the first conductivity type. The first semiconductor substrate of the first conductivity type and the diffusion region of the second conductivity type form a light detection photodiode portion. The diffusion region of the second conductivity type has a diffusion depth less than or equal to a penetration depth of short-wavelength signal light.
Therefore, a low-concentration (high-specific-resistance) epitaxial growth layer which is difficult to control is not used whereas a low-concentration substrate which is relatively easy to produce is used. A circuit-incorporating light receiving device having a circuit element having an optimized characteristic, and a light detection photodiode having a high sensitivity and a high-speed response over a range from long-wavelength to short-wavelength can be achieved.
In one embodiment of this invention, the diffusion depth of the diffusion region of the second conductivity type is less than or equal to 0.3 xcexcm.
In one embodiment of this invention, the short-wavelength signal light is blue light.
In one embodiment of this invention, a second semiconductor substrate of the first conductivity type is provided at the bottom side of the first semiconductor substrate of the first conductivity type, wherein a high-concentration buried layer of the first conductivity type having an impurity concentration greater than an impurity concentration of the first semiconductor substrate of the first conductivity type is provided in the second semiconductor substrate of the first conductivity type.
Therefore, an anode resistance in a traverse direction up to an electrode is reduced in the high-concentration buried layer. Further, a potential barrier is produced against carriers generated deeper than the buried layer when long-wavelength signal light enters the device. In this case, the carriers are prevented from contributing to a photoelectric current, thereby making it possible to prevent a reduction in a response speed due to a diffusion current. Furthermore, the substrates attached to each other is used whereas a low-concentration epitaxial growth layer which is difficult to control is used, and a low-concentration region can be provided on the high-concentration buried layer.
In one embodiment of this invention, a second semiconductor substrate of the first conductivity type is provided at the bottom side of the first semiconductor substrate of the first conductivity type, wherein the second semiconductor substrate of the first conductivity type has an impurity concentration greater than an impurity concentration of the first semiconductor substrate of the first conductivity type.
Therefore, an anode resistance in a traverse direction up to an electrode is reduced in the high-concentration buried layer. Further, a potential barrier is produced against carriers generated deeper than the buried layer when long-wavelength signal light enters the device. In this case, the carriers are prevented from contributing to a photoelectric current, thereby making it possible to prevent a reduction in a response speed due to a diffusion current. Furthermore, the substrates attached to each other is used whereas a low-concentration epitaxial growth layer which is difficult to control is used, and a low-concentration region can be provided on the high-concentration buried layer.
In one embodiment of this invention, the second portion of the first semiconductor substrate of the first conductivity type and the first portion of the first semiconductor layer of the first conductivity type have an N-type well region and a P-type well region, and the circuit element is provided using the N-type well region and the P-type well region.
In one embodiment of this invention, an isolation diffusion region of the first conductivity type is provided between the light detection photodiode portion and the circuit element, wherein an impurity concentration of the isolation diffusion region is greater than an impurity concentration of the first semiconductor substrate of the first conductivity type, and the isolation diffusion region reaches a position deeper than the light detection photodiode portion and the circuit element.
In one embodiment of this invention, an impurity concentration of the first semiconductor substrate of the first conductivity type is greater than or equal to 1xc3x971014 cmxe2x88x923.
In one embodiment of this invention, the diffusion region of the second conductivity type is divided into a plurality of regions by the first semiconductor substrate of the first conductivity type.
In one embodiment of this invention, the diffusion region of the second conductivity type is divided into a plurality of regions by a second groove.
In one embodiment of this invention, the first groove is provided by a LOCOS method.
Thus, the invention described herein makes possible the advantages of providing a circuit-incorporating light receiving device which can read from short- to long-wavelength signal light which is used for a CD, a DVD, or a blue-light DVD, and which can be easily produced by integrating circuit elements, such as a transistor, on the same substrate.